1. Technical Field of the Invention
The present invention relates generally to the field of signal processing, and, more particularly, to a method and apparatus for decoding a compressed video signal for use by another unit having a lower resolution, or alternatively, an equal or higher resolution than the compressed video signal.
2. Description of the Prior Art
Video image signals representative of video pictures are often processed at a first location (transmitter location) to encode the video image signals into a compressed video bit stream. The encoded bit stream may then be transmitted from the first location to a second location (receiver location) where the received bit stream is decoded for displaying the video pictures, processing, or storing the pixel values for later retrieval at the receiver location. The receiver location may, for example, process the decoded bit stream to code with a new compression format, or display the video pictures on a monitor or other display unit.
Video image signals may be displayed using a variety of video formats, such as common intermediate format (CIF) and quarter common intermediate format (QCIF). CIF specifies a data rate of 30 frames per second (fps), with each frame containing 288 lines and 352 pixels per line (352*288). QCIF, a related standard, also specifies a data rate of 30 fps, however, each frame contains only 144 lines and 176 pixels per line (176*144). QCIF is therefore one-fourth the resolution of CIF. Several other formats exist, e.g. PGA and MPEG, which provide a multiplicity of resolutions available for displaying, storing, processing, etc. a video signal.
It sometimes occurs that the unit for storing, processing or displaying at the receiver location has a different resolution than that of the compressed video signal to which the bit stream corresponds. For example, the bit stream may correspond to a CIF picture resolution, whereas the unit for displaying, storing, or processing at the receiver location might use a QCIF resolution. This resolution difference necessitates that a downscaling procedure be carried out at the receiver location to permit the display unit to properly display the lower resolution picture.
FIG. 2 schematically illustrates a video decoding procedure that is known in the prior art and that may be carried out in receiver processing circuitry. Basically, the procedure includes first decoding the compressed video bit stream corresponding to, for example, CIF resolution, and then downscaling the decoded signal in order to, for example, display the image on a monitor that uses a different resolution than the compressed video bit stream. More particularly, the compressed video bit stream 121 is decoded by first passing the signal through an inverse discrete cosine transform (IDCT) 126. Then the prediction block 128 provides motion compensation by applying the motion vectors to the previous compressed video bit stream to form a reconstructed image. After decoding, the image is downscaled to produce the lower resolution image. The image is passed through a low-pass filter (not specifically shown), followed by a sub-sampling block 124 which sub-samples the image to produce the lower resolution picture which can be stored, processed, or displayed.
In the system illustrated in FIG. 2 the signal received by the receiver apparatus is first decoded with full resolution. A downscaling process is then performed so that the picture will fit into the low resolution display of the display unit. Decoding with full resolution and then downscaling is a complex process which is quite demanding of both memory and CPU capacity in the receiver apparatus.